The favorite feline stars in Rosemary Wells's Yoko & Friends School Days series, which debuts with two titles, Mama, Don't Go! and The School Play, illus. by Jody Wheeler. In the first, Yoko won't let Mama leave the classroom until her classmate gives her courage. In the second, an unhappy Yoko is assigned the role of the cavity in the class play about dental hygiene, but an inspired idea creates sweet success. Simple text and familiar fuzzy-faced characters soothe the school-age set. (Sept.)